1. Field of the Invention
The present invention relates to a heat shield for a Czochralski type crystal growing furnace.
2. Description of Related Art
Photovoltaic cells may be constructed from silicon wafers that are cut from a single crystal ingot. Silicon ingots are typically created using one of two processes commonly referred to as the Czochralski (CZ) method and the float zone (FZ) method. In the Czochralski method a seed is placed in a crucible of melted raw silicon and then pulled away from the crucible. The surface tension of the silicon pulls molten material out of the crucible, wherein the silicon cools and solidifies in an area commonly known as the melt interface. The pull rate of the seed and the temperature of the molten silicon are closely controlled by a computer to insure that the ingot grows as a single crystal. Conventional crystal growing furnaces also provide gaseous jet streams that cool the ingot, and prevent oxidation and the diffusion of impurities into the silicon.
The production rate of silicon ingots is greatly dependent upon the pull rate of the furnace. The faster the pull rate, the greater the production rate. The pull rate is limited by the heat transfer rate of the ingot as the silicon is pulled out of the crucible. A significant amount of heat can be transferred to the ingot from the molten silicon through radiative heat transfer. To increase both the cooling rate and the pull rate of the ingot, it is desirable to provide a heat shield which greatly reduces the amount of radiative heat transfer from the molten silicon to the ingot. Such a shield must be capable of withstanding the extreme temperatures of molten silicon and not contaminate the ingot.
U.S. Pat. No. 4,330,362 issued to Zulehner, discloses a CZ crystal growing furnace which contains a radiation heat shield located directly above the molten silicon. The Zulehner heat shield is attached to an outer shield member that extends along the outer wall of the crucible. The silicon is typically replenished by raw silicon pieces that are fed to the crucible through a chute. Although effective in shielding the ingot, the Zulehner system requires a continuous feeding of silicon pieces through a chute system.
Most conventional CZ furnaces begin with a pile of raw silicon pieces that are then melted and pulled until an ingot is formed. The crucible is then refilled with raw silicon and the process is repeated. The pile of raw silicon is typically much taller then the molten silicon bath. The Zulehner system would therefore be impractical for conventional CZ furnaces, because the heat shield would not accommodate the change in height between the pile of raw silicon and the molten bath.
U.S. Pat. No. 4,330,361 issued Kuhn-Kuhnenfeld et al, discloses a CZ furnace which has a radiation screen that can be lowered and pivoted adjacent to the molten silicon after the raw silicon pieces have been melted. The Kuhn-Kuhnenfeld system requires more furnace space than a conventional CZ furnace because of the additional area required to pivot the heat shield. It would be desirable to have a radiative heat shield that was compact, simple to use and would compensate for the differences in height between the raw silicon and the molten silicon bath.